A desert is what is left after the mountains take their share

California's deserts are not where the storms refuse to go. They are where the storms arrive already spent — and the accounting can be done range by range.

Stand a mountain range across a moist airstream and it runs a toll booth. Air forced up the windward slope cools, condenses and rains; the same air descending the far side warms by compression, and warming air moves away from saturation rather than toward it. The windward slope gets a storm. The lee side gets wind, clearing skies, and almost nothing in the gauge.

That is the entire mechanism behind the driest third of California, and the part worth appreciating is how repeatable the arithmetic is. Blue Canyon, on the Sierra's windward slope, averages about 62 inches a year. Cross the crest to Bishop and the same storms deliver about five. Mammoth Lakes, sitting nearly at the crest, takes five times what Bishop gets a valley away. The pattern repeats at every scale: Idyllwild against Palm Springs across the San Jacinto crest, Julian against El Centro across the Peninsular ranges.

Stacking the shadows

Death Valley is what happens when the tolls stack. A Pacific storm bound for Furnace Creek crosses the Coast Ranges, then the Sierra Nevada or the Transverse Ranges, then the Panamints — each crossing drier than the last — and arrives with nothing left to give. Blue Canyon's annual total is about 28 times Furnace Creek's, and the two stations are watered by the same ocean and largely the same storms.

A shadow is not a desert

The rain shadow reaches far beyond the ground anyone calls desert. The Owens Valley, the Modoc Plateau and the whole Great Basin edge of the state are dry for exactly the same reason, without the heat that the word desert implies. Dryness is set by what stands between you and the Pacific; temperature is set by elevation and latitude. The Mojave is high and cold in winter, the Colorado Desert is low and almost frost-free, and Bodie, in the Sierra's shadow at 8,400 feet, is one of the coldest places in the state. All three are downwind of the same wall.